This invention relates to an ethylene-alpha-olefin rubber composition with improved damping characteristics which can also be molded, bonded to metal, and used as the vibration absorbing element in torsional vibration dampers, engine mounts, or other such vibration control devices. This invention also relates to a crankshaft damper for absorbing torsional and bending vibrations utilizing the improved rubber composition as the vibration absorbing element.
Rubber compositions are in wide use in vibration control devices. Diene elastomers such as NR, BR, SBR, IIR, CR and NBR have traditionally been used because of their low cost. They are generally vulcanized by means of heat-activated cure systems comprising sulfur and sulfur-based cure accelerators. Rubber formulated with these elastomers is generally very limited in terms of heat resistance and ozone resistance. As performance demands have increased in many applications, such as the increase in under-hood temperatures in automotive applications, higher performance elastomers such as EPM, EPDM, HNBR, AEM, fluoro- and silicone rubbers have increased in use. EPM and EPDM, members of the ethylene-alpha-olefin family of elastomers, are desirable for vibration dampers because of their high heat resistance, ease of incorporating fillers, and relatively low cost. EPDM and EPM are also desirable because they can readily be cured with peroxide cure systems which are known to provide better compression set properties, better heat resistance, and better compatibility with certain metal-adhesive coagents than sulfur cure systems. Unfortunately for some vibration damping applications, ethylene-alpha-olefin elastomers tend to be very resilient, low-damping elastomers.
For vibration damping, the most important property of the rubber composition is the degree of damping. One way to characterize the damping of rubber is to measure the ratio of loss modulus to storage modulus, known as “tan δ”, by dynamic mechanical testing. Typical tan δ values for EPDM compositions are in the 0.05 to 0.1 range in the typical temperature range of operation from room temperature to about 100° C. What is needed in some vibration control devices is a peroxide-cured ethylene-alpha-olefin composition with improved, higher damping and tan δ values, e.g., around 0.2 or higher. An increase in damping of around 100% or more may be desired.
In the article “Low Modulus, High Damping, High Fatigue Life Elastomer Compounds for Vibration Isolation,” Rubber Chemistry & Technology, 57(4) 792-803 (1984), authors M. A. Lemieux and P. C. Killgoar, Jr. describe their attempts to increase the damping of sulfur-cured NR compositions and NR/BR blend compositions for automotive suspension applications. Known methods of increasing damping include adding more filler, decreasing the amount of curative, changing the amount of plasticizer and changing or blending elastomers. Each of these known approaches has general limitations such as negative effects on other properties of the rubber or on the overall balance of properties of the rubber. Moreover, the particular utility, advantages and/or disadvantages of each approach are generally unpredictable without extensive experimentation.
After extensive evaluation of many compositions, ingredients and blends, the present invention unexpectedly meets the need for an ethylene-alpha-olefin rubber composition with increased damping for use in vibration dampers, in other vibration control devices, and in engineered rubber products subject to dynamic loading such as belts and hose. The improvement desired for vibration dampers is to at least increase the normal value of tan δ about 30% without degrading other desirable properties.
U.S. Pat. No. 6,386,065, which is incorporated herein by reference, discloses an example of a torsional vibration damper to which the subject invention could be applied. Additional examples of crankshaft dampers and examples of rubber compositions for vibration damping to which this invention could be applied are disclosed in U.S. Pat. No. 7,078,104, which is incorporated herein by reference. Reference is also made to U.S. Pat. No. 7,166,678 which discloses a different solution to the same problem.